1. Field of the Invention
This invention relates to microwave bandpass filters and to a method of operating said filters and, more particularly, to further substantial miniaturization achievable in dielectric resonator filters where mass and size of the filters are critical and to a method of operating said filters at cryogenic temperatures.
2. Description of the Prior Art
It is known that the use of dielectric resonators in microwave filters results in a significant reduction in size and mass while maintaining a performance comparable to that of waveguide filters without dielectric resonators. A typical dielectric resonator filter consists of a ceramic resonator disc mounted in a particular way inside a metal cavity. In addition to miniaturization, loss performance as well as thermal and mechanical stability are also important considerations in the design of dielectric resonator filters.
For dielectric resonator filters, the size of the cavity can be substantially reduced by mounting the dielectric resonator along a base wall of the cavity rather than mounting the resonator in a centre of the cavity. However, when the resonator is mounted on a base wall of the cavity, the Q of the resonator may be reduced and it may be difficult to mount the resonator inside the cavity. Further, current resonator mountings may not withstand the mechanical vibrations that are necessary when the filter is used in space applications.
Single, dual and triple mode dielectric resonator waveguide filters are known (See U.S. Pat. No. 4,142,164 by Nishikawa, et al. issued Feb. 27th, 1979; U.S. Pat. No. 4,028,652 by Wakino, et al. issued Jun. 7th, 1977; Paper by Guillon, et al. entitled "Dielectric Resonator Dual-Mode Filters", Electron Lett, Volume 16, pages 646 to 647, Aug. 14th, 1980; U.S. Pat. No. 4,489,293 by Fiedziuszko issued Dec. 18th, 1984; U.S. Pat. No. 4,453,146 by Fiedziuszko issued Jun. 5th, 1984; U.S. Pat. No. 4,652,843 by Tang, et al. issued Mar. 24th, 1987; U.S. Pat. No. 4,675,630 by Tang, et al. issued Jun. 23rd, 1987; U.S. Pat. No. 5,083,102 by Zaki issued Jan. 21, 1992.).
U.S. Pat. No. 5,179,074 was issued on Jan. 12th, 1993 to Fiedziuszko, et al. and describes a hybrid dielectric resonator high temperature superconductor filter which utilizes a plurality of resonators in a cavity where each resonator is spaced from a conductive wall of the cavity by a superconductive layer. The superconductive layer is capable of superconducting at temperatures as high as about 77.degree. K. A heat exchanger is used to maintain the housing at or below the critical temperature of the superconductor. The filter uses a heat exchanger or other temperature control means to cool the structure to maintain a superconductive sheet below the critical temperature for superconduction. The filter as described cannot achieve the standard of miniaturization achieved by the present invention and, among other differences, does not disclose the use of dielectric blocks. U.S. Pat. No. 4,881,051 by W. C. Tang, et al. issued Nov. 14th, 1989 describes a dielectric image-resonator multiplexer but does not suggest the structure of the present invention and is not capable of achieving the miniaturization achieved with the present invention.